Allow block and char classes in DeviceAllow bus properties (#4353)

Allowed paths are unified betwen the configuration file parses and the bus
property checker. The biggest change is that the bus code now allows "block-"
and "char-" classes. In addition, path_startswith("/dev") was used in the bus
code, and startswith("/dev") was used in the config file code. It seems
reasonable to use path_startswith() which allows a slightly broader class of
strings.

Fixes #3935.

src/basic/path-util.h

@@ -125,5 +125,6 @@ char *file_in_same_dir(const char *path, const char *filename);
 bool hidden_or_backup_file(const char *filename) _pure_;
 
 bool is_device_path(const char *path);
+bool is_deviceallow_pattern(const char *path);
 
 int systemd_installation_has_version(const char *root, unsigned minimal_version);